What is the reason for charging a $300 deposit? If they are concerned with theft or damages, $300 is nothing... If they put a much larger hold, I can see some rationale, but I don't think $300 really ahceives anything if there is an actual need to charge your credit card due to theft/damages. They do seem to be able to charge your credit card after your return regardless of the hold though - I got charged for tolls in NY weeks after I returned the car to National (and no credit hold was ever placed on my card at the time of rental).
I wonder if Emerald Club Members are exempt from the credit hold policy as National does not put a hold on my card but Enterprise does.